chore: remove orphaned auto-bump infrastructure

Delete the bumpVersionAction custom action (action.yml, bump-version.py,
Dockerfile, README) and the per-chart .bumpversion.cfg files for
keycloak, keycloakx, and mailhog. Nothing references these since the
automatic version-bump job was removed from the release workflow.

Also update the pull request template: it still told contributors that
version bumps happen automatically based on commit-message keywords
(major/feat/chore) and pointed at a scripts/bump-version.py path that
never existed. Rewrite it to reflect the current manual-bump policy.

Author: dominiquemetz

.github/actions/bumpVersionAction/Dockerfile

@@ -1,5 +1,7 @@
 <!---
 Thanks for wanting to contribute.
 
-Manual updates to the chart version are not needed any more. The version bumps are now based on commit messages. If you want to bump the major version include `major` in the commit message. For a feature release, include `feature` or `feat`. If you don't want to create a new release at all, include `chore` in all your commit messages. The default is a new patch release. For the specific keywords have a look at [the script](scripts/bump-version.py).
+Chart version bumps are done manually: when making a change that should
+be released, update the `version` (and `appVersion` if the bundled app
+changed) in the relevant `charts/<chart>/Chart.yaml` as part of your PR.
 --->